The principle and development trend of inkjet printing technology

The principle and development trend of inkjet printing technology

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First,  Overview

In recent years, with the development of electronic publishing and cross-media publishing technology, the market for traditional printing methods has been greatly impacted, and the development of digital printing technology is changing with each passing day. The current market positioning of digital printing is mainly short-run printing, instant printing, on-demand printing and variable data printing. Generally speaking, digital printing is a new type of printing service that can provide users with relevant services according to users' time requirements, location requirements, quantity requirements, cost requirements and certain specific requirements.

Inkjet technology has gradually become one of the mainstream technologies of digital printing. Inkjet technology is becoming more and more widely used, from desktop photo printers to digital proofing systems to digital presses, and is growing in every field: in the field of small format printing, inkjet printers are gradually becoming home and office. And commercial general-purpose products, the domestic market sales of millions of units; in the medium and wide format printing field, inkjet systems will become the mainstream of digital proofing and digital printing. Inkjet printing is advancing toward almost all areas of printing, and with the development of new consumables and improved technology, the quality of printing is getting better and better. As can be seen from DRUPA 2004, inkjet printing has become one of the technical highlights and continues to be one of the themes of the show at the just-concluded DRUPA 2008.

Second, the principle of inkjet printing technology

Inkjet technology uses a computer to directly control the output of the technology, no printing plate, no pressure, non-contact, with the common features of digital printing without printing, and can achieve on-demand printing and variable data printing. The principle of inkjet technology is to first transmit the color graphic information generated by the computer or the color graphic information from the prepress input device to the inkjet device, and then calculate the corresponding channel ink under the control of the computer through a special device. The inkjet image forming device controls the ink droplets to be ejected from the nozzle to the surface of the substrate at a certain speed, and finally, through the interaction of the ink and the substrate, the ink reproduces stable graphic information on the substrate.

Inkjet technology is available in various forms, and it can be divided into continuous inkjet and drop-on-demand inkjet according to whether inkjet is continuous. The ink jet technology used in industrial printing systems is mostly continuous jet type. For smaller ones, the on-demand jet type is often used.

The basic principle of continuous inkjet is shown in Figure 1. During operation of the device, the ink is continuously ejected from the nozzle by the drop generator, directed into the charge electrode to split into fine droplets, and simultaneously carrying the same symbol of charge; charged ink droplets enter the deflection The electric field, depending on its deflection amplitude in the deflection electric field, is captured by the ink droplet collector into the circulation loop and finally sent back to the ink droplet generator for reuse, or deflected to avoid the ink droplet generator eventually reaching the surface of the substrate, Form graphic information. The continuous inkjet system has the advantages of high frequency response and high speed printing, but the structure of the printer is complicated, requiring a pressurizing device, a charging electrode and a deflection electric field, and the terminal has an ink droplet recovery and circulation device in the ink circulation. In the process, it is necessary to provide a filter to filter impurities and gases mixed in, and thus it is difficult to mass-produce.

Figure 1 Schematic diagram of the basic principle of continuous inkjet

Continuous inkjet printing technology, which is widely used in high-speed digital printing equipment, is widely used. Several new equipments from multiple manufacturers are on display at drupa2008. At this exhibition, Kodak exhibited its newly developed Stream inkjet printing technology and its concept machine. Stream is a new concept of "thermal pinch-off" continuous inkjet printing technology, while adding 10 ~ 60nm particles in the ink, ink drying performance is unique, with faster speed and better The print quality, although not yet ready for industrial applications, represents the future of Kodak's inkjet printing technology.

The drop-on-demand inkjet technique is to eject ink droplets only in the portion of the image where ink ejection is required, while in the blank portion, no ink droplets are ejected. The spraying method does not need to charge the ink droplets, and the charging electrode and the deflection electric field are not needed. The nozzle structure is simple, the nozzle is easy to be multi-mouthed, and the output quality is finer; the pulse is controlled, and the digitization is easy; respectively, the yellow ink and the ink are selected respectively. The color head can be realized by the nozzle, and the colorization is easy; however, the ink droplet ejection speed is generally low. There are three types of thermal bubble jet, piezoelectric inkjet, and electrostatic inkjet, and of course other types.

The realization principle of the thermal bubble type inkjet technology is that under the action of the heating pulse (recording signal), the temperature of the heating element on the nozzle rises, and the ink solvent in the vicinity is vaporized to generate a large number of small bubbles, and the bubbles are heated during the heating time. The volume is continuously increased, and to a certain extent, the generated pressure will cause the ink to be ejected from the nozzle and finally reach the surface of the substrate to reproduce the graphic information. The basic principle of thermal bubble inkjet technology is shown in Figure 2.

Figure 2 Schematic diagram of the basic principle of thermal bubble inkjet technology

Piezoelectric inkjet technology is realized by placing many small piezoelectric ceramics near the nozzle of the print head, and the piezoelectric crystal is deformed under the action of an electric field. To a certain extent, the ink is generated by the energy generated by the deformation. Extrusion from the ink chamber and ejection from the nozzle, the graphic data signal controls the amount of deformation of the piezoelectric crystal, thereby controlling the amount of ink ejection. The basic principle is shown in Figure 3.

Figure 3 Schematic diagram of the basic principle of piezoelectric inkjet technology

The principle of electrostatic inkjet technology is: the electric field between the inkjet system and the substrate, the balance of the surface tension of the nozzle is changed by the image signal, and the ink droplet is ejected from the nozzle to reach the substrate under the attraction of the electrostatic field. The surface forms an imprint. The basic principle of electrostatic inkjet is shown in Figure 4. The size of the ink droplets generated by the electrostatic inkjet technology is much smaller than that of the nozzle, so it has a high resolution and is easy to realize the nozzle's multi-mouth; however, a higher operating voltage is required.

Figure 4 Schematic diagram of the basic principle of electrostatic inkjet technology

Compared with continuous inkjet technology, on-demand inkjet technology has the advantages of better quality and wider application field, although it is slower. At the same time, with the improvement and maturity of technology, the printing speed is steadily increasing. The recognition of drop-on-demand inkjet printing technology continues to increase. At the just-concluded Drupa2008, FUJIFILM Dimatix exhibited new technologies including Shaped Piezo Silicon MEMS technology, prototypes with piezoelectric drop-on-demand inkjet printheads and single-channel inkjet printing equipment. MEMS-based printhead products provide tighter control of the nozzle shape and more precise control of the absolute position on the silicon wafer, allowing for higher drop placement accuracy over longer jet distances. The rugged silicon construction makes operation more reliable and lasts longer, all of which are required to create new printers and open up new applications. It is also worth noting that Kodak introduced the Versamark VL2000, the first high-speed drop-on-demand inkjet printer in its history. The new Versamark VL2000 has improved color image quality while maintaining the same speed as the existing Versamark VT3000.

Third, the development trend of inkjet printing technology

The following is a technical perspective on the future development of inkjet technology.

1. Color control and management in the color separation process

Precise color management technology in the color separation process is the key to the quality of inkjet printing. The principle of color management technology based on ICC specification is shown in Figure 5.

Figure 5: Color management technology based on ICC specification to realize the conversion of input system RGB mode to inkjet system CMYK mode

The idea of color management based on the ICC specification is to use the CIE color system as a common language and standard for describing the color characteristics of various devices. It is compatible with other color management software or systems, and can realize color circulation and management between multiple processes; different color conversion methods can be flexibly selected according to actual needs. This facilitates uniform color control and management throughout the digital prepress and printing process. Of course, this color management idea also has certain flaws

1.1 chaotic gamut mapping: there are four different mapping methods (sensory, saturated, relative chromatic, absolute chromatic) for each property file, if two different property files are used Performing a color conversion will result in 4 x 4 = 16 blending options, which creates difficulties for the actual operation.

1.2 Color management module is not uniform: There are many softwares with color management functions on the market, but different color management modules are used inside each software, and the final result will be biased when color conversion is performed.

1.3 Black channel problem: In the process of color space conversion of different dimensions, the color precision loss is large, especially the K value, which is completely generated by mathematical operation, which is different from the K value on the printed proof.

1.4 does not apply to the needs of high-end professional: if the user requirements are high, it is necessary to continuously create optimized feature files, and to manually edit them, this process is more troublesome.

1.5 does not take into account the gain of the dot. The ICC-based color management technology does not take into account the gain of the dot, which has a great influence on the proof quality. If the dot expansion of the proof does not match the printed sheet, there cannot be an optimized simulated proof.

1.6 Calibration and characterization are not separated: printer calibration and characterization are not strictly separated. Once the equipment, materials, and process characteristics have changed, the profile must be regenerated.

1.7 Not suitable for remote operation: Since calibration and characterization are not always performed separately, it is required to re-characterization by highly experienced employees at the remote end, but often the actual conditions are not available and this will increase the cost.

These have become factors that currently affect the conversion accuracy of the input system RGB mode to the inkjet system CMYK mode. It is necessary to perfect the ICC color management idea or to establish a new color control and management model.

In fact, with the continuous development of science and technology, people have been working hard to study new color management systems. According to the data, the color management system WCS (Windows Color System) is adopted by Microsoft's newly launched Windows Vista operating system. Based on the color appearance model CIECAM02, WCS will provide more realistic screen effects, more comprehensive color display, and more realistic printing technology. With the use of new tools, it can better support related hardware such as digital cameras and color printers. WCS is considered to be a dynamic color management system. It uses a new feature file structure. The feature file is written in Extensible Markup Language (XML), which simplifies the process of creating, verifying and editing feature files. WCS uses color appearance. The model CIECAM02 is a standard color space for gamut mapping, which can reproduce color attributes of different visual conditions in a certain environment. At the same time, it also supports 16-bit and 32-bit more precise colors, which can include a wider range of colors. When the user performs color conversion, the appropriate gamut mapping model and the visual model of the source device and the target device can be selected according to specific requirements; WCS has better scalability and compatibility.

For the first time, WCS introduced the color appearance model into the color management system, breaking through the existing color management methods and establishing a new color management concept, enabling users to predict the color effects under different visual conditions and solve this long-term problem. A problem that plagues people. Color management technology based on color appearance model will have a great impact on modern color management technology, and also pointed out the direction for the development of future color management technology.

Now people have begun to try to apply the new color model iCAM to color management to achieve color management of cross-media, cross-platform and high dynamic range color reproduction process. With the deepening of people's research, it is believed that iCAM is even more perfect. The color management system of the color appearance model will be developed soon, and the color management technology will be more perfect. The research and application of new technologies and the development of new products will be waiting to be seen.

In addition, high-fidelity color separation and screening technology will further promote the color management and control of inkjet technology and the improvement of print quality. High fidelity printing technology is relatively four-color printing and printing technology. In four-color printing, four colors of C, M, Y, and K inks are used for printing. In theory, it is enough to use C, M, and Y three-color inks. But in fact, due to various reasons of the process, it is also necessary to increase the black ink. Similarly, the secondary colors superimposed on the two basic colors are also unsatisfactory, and it is necessary to add a secondary color spot color ink. For example, the red color produced by the overlapping of the two inks of M and Y is not as bright as the special red ink. Under the premise of this theory, high-fidelity printing is produced, that is, several spot color printings are added on the basis of four-color printing to expand the color expression ability of printed matter. The conventional color separation technology divides the color of the original into three basic colors of cyan, magenta, and yellow, and adds black to form the chromatographic range that the print can represent. This range (called color coverage or coverage umbrella) is much smaller than natural colors. In other words, the rich colors of nature can't be expressed with only four kinds of inks. One of the purposes of making prints in six-color, seven-color, and eight-color high-fidelity colors is to expand the range of color reproduction and more realistically reflect nature. Since the prints using more than four colors for color separation printing cannot be reproduced by conventional four-color separation technology, this technology has a special status in high value-added prints.

2. Inkjet print head design

Inkjet printhead technology is very important, and the great development of inkjet technology is mainly due to the development of the principle of inkjet printhead technology.

The continuous inkjet system has the advantages of high frequency response and high-speed printing. However, the inkjet printhead structure of this method is relatively complicated, and requires a pressurizing device, a charging electrode, and a deflection electric field, etc., which is difficult to mass-produce.

The inkjet print head of the drop-on-demand inkjet system has a simple structure, is easy to realize the multi-mouthing of the inkjet head, is easy to digitize and colorize, and has fine image quality, but generally the ink droplet ejection speed is low.

The total number of thermal bubble jet nozzles can reach hundreds or even thousands, and the nozzles are quite fine, producing a rich blend of colors and smoother mesh. The ink cartridge and the nozzle form an integrated structure. When the ink cartridge is replaced, the inkjet head is updated at the same time, so that there is no need to worry about the nozzle clogging, but the waste material is wasted and the cost is relatively high.

Due to the high operating temperature of thermal bubble jets, generally greater than 300 ° C, there is often an insoluble matter deposited on the heating electrodes (these deposits are inorganic salts, mainly from inorganic impurities in the ink; there are also organic compounds, mainly from The thermal decomposition product of the ink), so that the vaporization of the heating electrode is lowered, the number of ink droplets is small, and the volume of the ink droplet is reduced. In severe cases, the printing head cannot work normally, which affects the service life. In addition, the ink is prone to chemical changes at high temperatures, the properties are unstable, and the print quality is affected to some extent. On the other hand, since the ink is ejected through the bubbles, the directionality and volume of the ink particles are very poor. The edges of the printed lines are easily staggered, which also affects the print quality to some extent.

Piezoelectric inkjet technology effectively adjusts the size and usage of ink droplets by controlling the voltage. Compared with thermal bubble inkjet technology, piezoelectric inkjet technology has relatively regular ink droplet shape, controllable droplet size and ejection speed, no sputtering, accurate positioning, more types of inks, and inkjet head life. Relatively long and other advantages. Piezoelectric inkjet technology can also use pigment inks to prevent discoloration and fading caused by ultraviolet radiation. The ink jet head made by the piezoelectric ink jet technology has a high cost. In order to reduce the user's use cost, the print head and the ink cartridge are generally separated, and it is not necessary to replace the print head when replacing the ink. A disadvantage of piezoelectric inkjet technology is that once the nozzle is clogged, the cost of dredging or replacement is high and it is difficult to operate.

The micro-piezoelectric inkjet head technology developed on the basis of piezoelectric technology has stronger control ability for ink droplets, more regular shape of ink droplet particles, more accurate positioning, improved printing resolution, and easier realization of high-precision printing. Piezo inkjet-based printing technology has the greatest potential to challenge traditional printing processes.

Hot bubble inkjet technology and piezoelectric inkjet technology have their own advantages and disadvantages. Currently, inkjet devices on the market are based on these two technologies. The use of thermal bubble inkjet technology is mainly for products of companies such as Canon and Hewlett-Packard, and piezoelectric inkjet technology is mainly used by Epson.

Although drop-on-demand inkjet technology is less efficient than continuous inkjet technology, and each on-demand inkjet head technology has its own drawbacks, as technology improves and new materials are developed, on-demand The speed of inkjet systems has been greatly improved, and each inkjet printhead manufacturer is constantly improving its products to make it more suitable for the needs of the market.

For example, FUJIFILM Dimatix, the world's leading supplier of piezoelectric drop-on-demand inkjet products, its piezoelectric drop-on-demand inkjet printheads have excellent high-frequency inkjet and the ability to spray a wide variety of liquids. The characteristics are not at the expense of the accuracy of the drop point of the droplet. The printing accuracy at the production speed enables each nozzle on the print head to be printed at a high speed without affecting the accuracy of the drop placement. It also has high duty cycle and long life characteristics.

In order to improve the output quality, in addition to improving the inkjet head technology, many manufacturers are also making a fuss about ink droplet modulation. Micro ink droplet adjustment technology, smart ink droplet conversion technology, fine photo printing technology and other technologies have been adopted to inkjet output devices. Among them, the output quality of the inkjet system is greatly improved.

3. Inkjet printing paper

The characteristics of inkjet printing determine that its substrate can be varied, such as paper, plastic and metal. The basic characteristics required are fast ink absorption and non-diffusion. Should also have the following properties: 1 to have good recording performance, strong absorption capacity, fast absorption, high resolution, printed images are clear and beautiful, uniform color, printing effect is ideal; 2 record speed, spray The ink can evaporate quickly and dry, avoiding the spread of ink dots on the surface of the substrate. 3 For inkjet printing paper, in order to prevent the ink dots from spreading on the paper, coated paper or glossy photo paper is generally used, which requires a smooth surface and a certain degree of sizing, which can absorb ink quickly and avoid light. Scattering. The coating is required to have a certain degree of fastness and strength, and is not easily scratched; 4 good preservation, water resistance, light resistance, and fading.

There are three types of inkjet printing papers commonly used on the market: ordinary inkjet printing paper, inkjet printing coated paper, and microporous inkjet printing coated paper.

Ordinary inkjet paper is a low-end print medium that is used in home or small office inkjet printers at a lower price. Since it is not specially coated, it is only suitable for monochrome printing. When high-quality color printing, especially photo printing, is required, it is easy to cause problems such as penetration and stickiness, which seriously affects the quality of inkjet printing.

The receiving layer of the inkjet printed coated paper can effectively interact with the coloring agent to increase its adhesion performance and obtain high printing chromaticity; the phenomenon of mutual penetration of blurred boundaries does not occur at the junction of different colors; the water resistance and light resistance of the graphic are also very good. Great improvement.

The microporous inkjet printing coated paper has a special microporous structure, and the coating has a strong ink absorbing force, and even a portion with a dark color tone can exhibit a good layering; the drying speed is fast; The material is very delicate, not only high in brightness, but also able to match high-precision photo printing; it has good water and light resistance.

4. Inkjet printing ink

Ink is the main consumable for inkjet printing systems. The type of ink determines the substrate, printability, and printing speed that an inkjet printing system can use. The ink for inkjet printing is a special ink with moderate viscosity. It should have the following properties: 1 non-toxic, non-clogging nozzle, good sprayability, non-corrosion to the metal parts of the nozzle; 2 for hot bubble inkjet systems, heat required Good stability, because the working temperature reaches 300-400 ° C, if the ink is not resistant to high temperature, it will decompose or change color; 3 must have sufficient surface tension to prevent the ink from spouting when it is ejected; 4 has fast drying to prevent The ink spreads over the substrate.

There are two main types of inks currently available for inkjet printing, one is defined as a non-solid ink; and one is a full solids ink.

Among the non-solid inks, it is divided into water-based inks and solvent inks. Water-based inks use pigments or dyes as color formers, and the other main raw material is water. Water-based inks are commonly used in inkjet printers in homes or small offices, relying on their penetration and absorption capabilities to complete the entire drying process, although this process is accompanied by moisture volatilization, but overall the drying rate is relatively slow. The nature of the ink itself reduces the color density, and because the ink penetrates deep into the substrate, it reduces the resolution of the ink on the paper.

Compared with water-based inks, solvent-based inks have a fast drying speed. The inks of such inks are relatively glossy. The inks adhere to the substrate after drying, and are generally not sensitive to the temperature and humidity of the environment. However, organic solvents in solvent-based inks can cause environmental pollution. Environmentally friendly solvent-based inks have emerged, replacing non-toxic and non-polluting solvents with common solvents.

When printing or printing on inkjet paper or coated substrates, the use of aqueous and solvent-based inks, image quality and durability are acceptable, but when in poorly absorbable substrates (such as metal, glass and plastic) When you print or print, you may not get a long-lasting, clear image. In order to solve this problem, a full solid content ink was introduced.

There are two main types of solid inks, phase change inks and UV UV curable inks. Phase change inks have no components that require volatilization and penetration and are very fast drying. Its composition is very simple, the main connecting agent is a low melting point polymer.

Another is UV UV-curable ink, which was developed at the beginning of this century. The main constituent linker is the prepolymer + monomer + initiator. UV-curable inks are highly printable and can satisfy the needs of a variety of substrates. The images printed with these inks have stronger durability and non-fading ability, and the reliability and stability of the print head have never been seen before. Improvement. In addition, UV-curable inks have excellent drying properties and good environmental performance. UV curing ink combined with digital inkjet technology can bring high economic benefits. From the technical point of view and economic point of view, UV curing ink has a broad application prospect and becomes the development direction of inkjet printing ink in the future.

Fourth, the development trend of inkjet printing applications

In the field of inkjet printing or printing applications, to sum up, there is only one sentence, that is, ubiquitous. There are probably a few typical application areas.

1. Large format color inkjet printing market

Large-format color inkjet printing is a non-printing digital printing technology, which is called large-format color inkjet printing. It is used to input graphic information stored in an electronic computer to an inkjet printing device. The micro-nozzle sprays the ink onto the surface of the substrate, and the adhesive is adsorbed by the ink and the substrate, thereby completing the reproduction printing of the graphic on the surface of the substrate.

Large-format inkjet printing has the following advantages: 1 digital input and output mode; 2 large printing format; 3 is not limited by printing; 4 diverse printing materials. With the development of printing technology and market economy, the market for large format printing will be more prosperous, and the application of large format inkjet printing technology will be more extensive. In the large format printing market, large format inkjet printing technology will seize some of the market share of screen printing and even large format offset printing, becoming a mainstream printing form.

2. Digital color proofing (including contract proofing) market

The digital proofing system can be divided into three grades, namely, a high-end mesh-based digital proofing system based on sublimation technology, a medium-quality inkjet digital proofing system, and a popular digital proofing system using an inkjet printer as a printing device. Due to the comprehensive consideration of cost, speed and quality, especially in the case of continuous development and improvement of inkjet printing technology, inkjet digital color proofing will also occupy an increasing share in the proofing market.

3. High-speed on-demand and variable digital printing market

Print On Demand (POD) is a new transformation of the entire publishing industry through digital and ultra-high-speed printing technology to meet the needs of different time, place, quantity and content to adapt to individualization, short version and high efficiency. The modern market demand is especially suitable for some printings with narrow orientation, strong professionalism, strong variability and small batch size. On-demand printing is a new printing process that combines advanced digital, technology and toner-based printing technologies.

Variable data printing, also known as Variable Data Printing (VDP), is an important part of digital printing. Some or all of the content (data) of each printing surface (printing surface) of variable data printing is variable, that is, during printing, the printed image or text can be changed according to preset contents and formats. So that the first to last prints have different images, texts or barcodes, and each print can be designed and printed for its specific issue.

With the advancement of science and the development of society, the short-run on-demand printing market is expanding day by day, and people's individualized demands are getting stronger and stronger. Under such circumstances, the market scope of digital printing technology is getting bigger and bigger, and many of them will be seized. It belongs to the market of traditional printing. The advantages of inkjet digital printing technology, due to cost and speed, and increasing quality, will also seize the opportunity of development and play a greater role in the market.

4. Output of digital color photos

Silver salt photographic paper is now equipped with a laser scanning device for rinsing. But after development, digital inkjet will likely become mainstream equipment. Now many manufacturers, such as Epson and Hewlett-Packard, are desperately competing in this field, because this market will undergo radical changes in the future, and everyone hopes to occupy a place in this market.

5. Home/office digital color output terminal

There are two options for this terminal, one for toner and one for inkjet. It can be said that inkjet printing is the mainstream way.

6. Textile printing and other fields

The textile printing field has long been used, and there should be great development in the future. In other areas that we have thought of and have not yet thought of, perhaps inkjet will play an increasingly important role.

V. Conclusion

Perhaps the biggest advancement in inkjet technology in recent years has been the introduction of drop-on-demand inkjets and UV-curable inks. Compared to traditional processes, inkjet technology is a digitally driven process that has all the advantages of digital processes, such as instant printing and variable data printing, to meet the needs of personalized printing, enabling on-demand printing. These advantages have become the main reason for the use of inkjet as a future production technology in various industries.

Due to the advantages of inkjet printing itself, as well as the development of related technologies and the development of related materials, inkjet technology has shown great potential in the field of small-format desktop printing, in the form of digital proofing and cost-effective short-run printing. The application field has also made great progress. In terms of digital proofing and digital printing format, wide-format inkjet technology will continue to expand its market for a long time to come. In the next few years, inkjet printing technology will continue to develop in the direction of product serialization, high speed, high quality, and low price.